1. Field of the Invention
This invention relates generally to the deep drawing of metal products, and is concerned in particular with an improved apparatus for progressively drawing cup-shaped metal blanks into elongated cylindrical tubes.
2. Description of the Prior Art
With reference to FIG. 1, the present invention is particularly suited for, although not limited in application to, the drawing of tubular barrel and cap sections 10, 12 of writing instruments 14. Such components typically are drawn from a "gold filled" material, which may consist of a base layer of copper composition, with nickel and gold layers superimposed thereon, each layer being bonded to the next adjacent layer.
With reference to FIG. 2, the drawing operation usually begins with a relatively shallow cup-shaped workpiece W.sub.a. As the workpiece progresses through a succession of drawing stages, each involving the use of a cylindrical plunger to force the workpiece through a circular die, the workpiece undergoes a gradual lengthening and reduction in diameter as depicted at W.sub.b -W.sub.e, with the finished piece being shown at W.sub.f.
During each drawing stage, the various layers of copper, nickel and gold flow at different rates with respect to each other and in response to different forces acting on the plungers. These variable flow rates and forces change from one drawing stage to the next as the shape of the workpiece changes.
In the past, it has been known to mount the plungers of the successive drawing stages on a common pivotal arm, the latter being operated by a master ram capable of delivering the total force required to effect simultaneous drawing at each stage. The problem with this arrangement, however, is that the plungers move at rates and in response to forces which are fixed in relation to each other, and thus not always suited to the metallic flow requirements at each drawing stage. As a result, drawn products often fail to meet specified tolerances. Where the rate of plunger movement is too rapid, surface cracks may develop in the outer gold layer or in the underlying copper or nickel layers, thus creating flaws which render the workpieces unsuitable for subsequent high tolerance forming operations. In extreme cases, the workpiece may be pierced prematurely at one of the intermediate drawing operations.
A further drawback with the prior art drawing systems stems from the workpieces being subjected to substantial frictional contact with the transfer mechanisms used to shift them from one drawing stage to the next. This often produces surface scratches which disfigure the products and render them unsuitable for their intended uses.